This invention relates to an oxygen sensing element of the type having a solid electrolyte oxygen concentration cell formed as a laminate of thin layers on a ceramic substrate, in which a heater is embedded and to which lead wires are attached.
In the automobile industry it has become familiar to install an oxygen sensor in the exhaust system as a means for detecting actual air/fuel ratio values in the engine. In most cases the oxygen sensor is of the oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte such as zirconia stabilized with calcia or yttria. In this field, a recent trend is to miniaturize the oxygen sensitive element of the sensor by constructing it as a laminate of thin, film-like layers on a ceramic substrate of very small size. The principal part of the laminate is a solid electrolyte layer and two electrode layers, namely, a reference electrode layer and a measurement electrode layer, formed adjacent the solid electrolyte layer. Usually an electric heater is embedded in the substrate to maintain the oxygen sensing element in operation at a sufficiently elevated temperature such as 600.degree.-800.degree. C. because the solid electrolyte concentration cell cannot properly function at temperatures below a certain level such as about 400.degree. C.
An oxygen sensing element of this type has a plurality of lead wires connected respectively to the heater and the reference and measurement electrode layers. These lead wires are fixed to the ceramic substrate by embedding their end or tip portions in the substrate. To ensure electrical connection of each lead wire with a conductor of the heater or one of the electrode layers, usually a hole is formed in the substrate for each lead wire at a location where the end portion of the lead wire makes contact with the heater or electrode terminal, and the hole is filled with a conductive paste which is subsequently sintered to turn into a solid conductor.
Since an oxygen sensing element may be used to sense gas concentration in a combustion gas such as the exhaust gas of an automotive engine, it is inevitable that some carbon or certain carbonaceous substance will be deposited on the substrate surface where a surface of the aforementioned conductor in each hole is exposed. Sometimes the deposited carbon will provide a conductive path between a conductor and a lead wire through which a current flows to the heater and another conductor extending to a lead wire for one of the electrode layers. When current is supplied to the heater, the establishment of such a conductive path has an unfavorable influence on the accuracy of any measurement of electromotive force generated by the oxygen concentration cell. Moreover, particularly when a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer is produced by externally supplying a DC current of a controlled intensity to the solid electrolyte layer as described in U.S. Pat. Nos. 4,207,159 and 4,224,113, any leakage of the heater current through the conductive path produced on the substrate surface is liable to cause a great increase in the current flowing through the solid electrolyte layer and, hence, a great increase in the magnitude of the reference oxygen partial pressure. As a result there arises a possibility of peeling of the reference electrode layer from the solid electrolyte layer or breaking of the solid electrolyte layer, whereby the oxygen sensing element tends to suffer from an unexpectedly short service life.